Looking Up : Moon’s long shadow to touch the Earth again

Friday

Jul 7, 2017 at 1:38 PMJul 7, 2017 at 1:38 PM

Peter Becker More Content Now

Much is being said about the total solar eclipse on August 21, 2017. This will be an extremely rare opportunity for Americans to enjoy the spectacle of the Sun being completely covered by the Moon, certainly one of the most awesome celestial events visible to the unaided eye.The darkest part of the Moon’s shadow, called the umbra, extends behind the moon, in a long cone. The shadow reaches out into space approximately 235,000 miles. Usually, in its orbit around the Earth, the Moon’s shadow completely misses us, since the Moon’s orbit is not exactly aligned with the Sun and Earth.On rare but predictable occasions, the planet catches the tapered shadow cone, and we have a total solar eclipse. If you are on that part of the planet when the Moon’s shadow hits, you will observe a most startling scene. Certainly if you have witnessed this before, or have at least heard or read about it, you can think you are prepared, but each time the event mesmerizes us, as what we come to expect from daytime turns upside down.The shadow of the Moon on the Earth is a round patch, round because the Moon is round. It is not sharply defined, but blurs at the edge.The Moon is approximately 2,100 miles wide, but because the umbra shadow tapers like a cone, when the shadow hits the Earth, it is only about 170 miles wide.The shadow races along in a path at nearly three times the speed of sound, averaging 2,288 miles an hour. The Moon is constantly moving west to east. Meanwhile, the Earth is moving underneath the shadow, as it spins from west to east at about 1,000 miles an hour at the equator.Each eclipse is not the same. How wide the umbra will be varies depending on how close or far the Moon happens to be. Other factors include where on Earth the shadow strikes and the elliptical orbit of the moon. The duration of the eclipse varies both along the track it takes and from one eclipse to the next.At the August 21st eclipse, the total phase will last up to nearly two minutes, 40 seconds, depending on where you are situated. This is the first coast-to-coast total solar eclipse in 99 years.On either side of the narrow, total eclipse path, the Moon partially blocks the Sun but never fully covers it. In this area the lighter “penumbra” shadow falls on the Earth. The rest of America will witness only a partial solar eclipse. Although thousands of people can be expected to plan vacations around eclipse time and try and be in the path of totality, millions more will stay at home, at their jobs or whatever they do. Hopefully all will have a chance to see even the partial event, for even a brief time.It is absolutely essential to do this safely. The Sun is overwhelmingly brilliant and should NOT be looked at even briefly, eclipse or no eclipse, without special protection. There are completely safe solar filters that block out the harmful ultraviolet and infrared rays, as well diminish the brilliance enough so your eyesight will not be damaged. There are also wonderful, indirect ways to witness a partial solar eclipse. More about this will be written in an upcoming column.During totality, however, there is no harm to your eyes, since the Sun is blocked out! Only the glowing corona of the Sun is visible around the edge of the blackened Moon, and this poses no danger to the eyes.Meanwhile, there are thousands of other stars besides the Sun to enjoy. Any one of these stars would also be harmful to the eyes to look at, if you were up close. Because of the incredibly vast distances, we can safely enjoy the starry night. The Sun too, would be like any one of these other stars, a bright point of light if we were just a few light years away (A light year is a measure of distance, not time. It is the distance light travels in a year, or about 5.8 trillion miles.) From Alpha Centauri, the Sun’s nearest stellar neighbor, the Sun would appear as a 0-magnitude star, similar to the bright star Vega seen high in the east on a July evening.Solar eclipses occur only when the Moon is at “New Moon”, the phase when it is between, or very nearly between, the Earth and Sun. Full Moon occurs July 8. There is one New Moon coming up, July 23, before eclipse time.Let’s hope for clear skies.Keep looking up!For more about the eclipse, visit: https://eclipse2017.nasa.gov/

— Peter Becker is Managing Editor at The News Eagle in Hawley, PA. Notes are welcome at news@neagle.com. Please mention in what newspaper or web site you read this column.